Basically even though the tasks are short term, the final desision is a long-term one.
While Yasmina may have helped to win the task saving herself for at least this week, by skimping so much on the quality of food, something that she touts herself as a expert on she may have done herself a bit more long-term harm, something that may come up when she's on the firing line later.
Take Sara for instance in S4, while not the most obvious candidate for the win, the fact that she 'appeared' to contribute nothing really to the other tasks hurt her in the long-term.
Or Alex in S4, each week he survived by a game-play strategy which may have helped him in the short run going by week-by-week. However when it came down to the final all the game-playing in the world couldn't help him face a more long-term idea of a task.